This invention relates generally to apparatus and methods for producing intumesced or puffed particles and more specifically relates to apparatus useful in the continuous production of intumesced particles as for example intumesced alkaline metal silicates, borax, perlite, vermiculite and the like.
Intumesced or puffed mineral particles having a generally spumiform nature, i.e. being of frothy or foamy characteristics, have found wide applications in numerous fields of commerce and technology. One such type of particles takes the form of a puffed intumesced or expanded alkaline metal silicate, and a number of issued U.S. patents can be cited pertinent to such subject. U.S. Pat. No. 3,756,839 for example, describes the preparation of silicate-based materials which may be intumesced, which are prepared from anhydrous silicates and insoluble powders and water. Other prior art patents relating to expandable silicate particles include U.S. Pat. Nos. 4,203,773; 3,728,208; and 3,765,919. The methods used to convert the intermediate or unpuffed such silicates to the expanded or intumesced form, commonly can include heating. For example, in the aforementioned U.S. Pat. No. 3,728,208, pellets are initially formed from dehydrated sodium silicate which is admixed with liquid sodium silicate, a mineral extender and oleophilic-hydrophobic agent. The pellets in semi-solid form are passed through a heating chamber such as a rotary kiln or a microwave oven, where at elevated temperatures in the range of about 250.degree. to 1000.degree. F. the pellets will expand into random shaped foam particles. During this heating process the volume of the pellets increases three to four times. These particular pellets are useful in oil spill control and removal; but expanded silicate particles are more generally useful in many applications, including as insulation, in construction applications, in packaging, and in other fields in which lightweight and chemically relatively inert particles are highly desirable.
Another type of intumesced or puffed particle of interest to the present invention is puffed borax. Reference may be had for background concerning this type of product to the present inventor's U.S. Pat. No. 4,412,978. In the preparation of puffed borax, particulate borax pentahydrate particles are rapidly heated to above the melting temperature of the pentahydrate, whereat the borate dissolves in its own water of hydration and the solution erupts through the partially dehydrated crystal surface resulting in the puffed product. The resultant product thus consists of low density particles or "beads", which have high surface area and include large quantities of voids. By virtue of such structure large quantities of liquids and/or solids can be loaded onto the expanded borax, rendering such products useful among other things as a bulk carrier in numerous applications.
As pointed out in my aforementioned 4,412,978 patent, various techniques have been utilized in the past to effect the desired heating of the borax feed material. U.S. Pat. No. 4,031,354 to D'Souza for example describes a rotary inclined tubular dryer into which granulated borax is fed at its higher end and puffed borax is discharged at the lower end. A countercurrent flow of dry air is maintained in the dryer. Borax is prevented from adhering to the interior of the tube by an adjustable spring-loaded scraper. In this process undesirable clusters or chunks tend to be formed. Other methods for puffing the borax are described in British Patent No. 629,171, and by use of a so-called borax "gun" one type of which is disclosed in U.S. Pat. No. 3,454,537, wherein the borax feed particles are mixed with hot gases in a venturi feed zone. Another approach is described in U.S. Pat. No. 3,822,034 to E. J. Gibbons in which the borax feed is introduced downwardly into an upwardly moving air stream in a spray tower.
In my own U.S. Pat. No. 4,412,978, a particularly advantageous method is used to produce a puffed borax of high strength and other improved characteristics. In the method described, particles of borax are introduced into a downwardly flowing, slowly moving laminar airstream in a vertical tower, as a horizontally dispersed sheet of particles. The air stream is heated sufficiently to remove part of the water of hydration without fusing the borax. The borax particles carried by the airstream are gravitationally separated downstream and are found to have a more uniform puffing ratio.
Another particle type of particular interest to the present invention is exemplified by the mineral vermiculite. Particles of this mineral under the action of heat can be expanded by exfoliation to result in materials useful as heat insulators or for many other purposes. Various apparatus have been described for this purpose, including that set forth in U.S. Pat. No. 2,139,378.
Yet another example of an expandable particulate material is perlite which is a volcanic glass of concentric onion-like structure. A furnace useful in expanding particles of this type is described in U.S. Pat. No. 2,550,877. Also see a further apparatus for such use in U.S. Pat. No. 3,097,832 as well as U.S. Pat. No. 3,206,905.
It will be apparent from the foregoing references that numerous of the prior art apparatus and techniques for expanding intumescent particles employ air as the heat transfer medium. One of the objections to this technique is that with certain types of feed particles (e.g. borates) such medium inherently captures many of the fines which invariably result because a commercially useable feed has a wide particle size distribution (PSD). This use of air as a transfer medium causes these fines to be carried by the air stream and they must be subsequently separated from the airstream and either discarded or added back to the finished product stream. Discarded fines add significantly to the cost of production and decrease yield, and adding them back to the main product stream results in a degradation of the finished product quality since large amount of fines cause dusting problems with subsequent handling and/or compounding.
Indeed, insofar as the production of intumesced particles from a feed having wide PSD is concerned, even processes and apparatus which do not use air as the heat transfer medium, e.g. rotary kilns and the like, do not in most instances remove the fines invariably produced with an airstream.
Regardless of the particular heating media or approach that may be used in the course of preparing the intumesced particles, a crucial concern is one of accurately controlling the degree of heating of the feed particles. The prior art apparatus have not been well adapted for these purposes. For example, while the flow of particulate matter through a spray tower does provide a very gentle environment for the heating action, it is difficult to accurately control the residence times of the particles, due to differences in flow rates across the interior of the tower, as well as due to differences in the particulate size which influences the drag forces on the particles. The range of possible dwell times in such towers is very limited, because the height of the tower is fixed, and accurate control of longer dwell times cannot be achieved by simply changing air flow.
The bulk of other furnace constructions which have heretofore been used for these purposes are similarly devoid of accurate dwell time control for the particles, in that in most instances it will be found that such particles progress through the furnaces in a rather indeterminate manner so that in general it is only the input/output movement from the furnace which progress with regularity, not the movement of individual parts of the particulate mass. It will be appreciated that in order to provide an accurate residence time in the heating portion of the device or furnace, the flow through the heating means must be accurately controlled; and moreover, the feed rate to the heating device must be accurately controlled as well, so that effectively there is a constant flow through the heater of uniform packets of material. These factors have not been adequately considered in the prior art, with the result that there has not to the present time been apparatus available which can meet all of the stated objectives.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide apparatus for continuous production of intumesced particles, which enables accurate and fully repeatable control of the heating of the packets of particles advancing through the apparatus, as well as accurate and complete control in the feed of the particulate matter into the heating portion of the device.
It is a further object of the invention to provide apparatus which will maintain particles stationary in relation to one another during intumescing to allow smaller particles or fines to be taken up by larger particles during the somewhat plastic intumescing stage.
It is a still further object of the present invention to provide apparatus of the aforementioned character which is highly adaptable to use with a variety of particulate materials, therefore rendering it useful in the intumescing or puffing of diverse particles, such as alkaline metal silicates, borates and mineral particles such as perlite, vermiculite and the like.